1. Field of the Invention
The subject invention relates to systems for reducing deposition of fluid-borne particles, for protecting surfaces from such particles, and for protecting surfaces from such particles while the surfaces are being changed structurally, such as during manufacture of integrated circuits, while such surfaces are being protected against deposition of fluid-borne particles thereon. The subject invention also relates to articles made by processes which include protection of these surfaces during such processes.
2. Information Disclosure Statement
The following disclosure statement is made pursuant to the duty of disclosure imposed by law and formulated in 37 CFR 1.56(a). No representation is hereby made that information thus disclosed in fact constitutes prior art, inasmuch as 37 CFR 1.56(a) relies on a materiality concept which depends on uncertain and inevitably subjective elements of substantial likelihood and reasonableness and inasmuch as a growing attitude appears to require citation of material which might lead to a discovery of pertinent material though not necessarily being of itself pertinent. Also, the following comments contain conclusions and observations which have only been drawn or become apparent after conception of the subject invention or which contrast the subject invention or its merits against the background of developments which may be subsequent in time or priority.
The soiling of indoor surfaces due to the deposition of airborne particulate matter is a commonly observed phenomenon. Of particular concern is the potential for damage to paintings and other works of art. The aesthetic quality of such materials may certainly be deteriorated by an accumulated deposit of airborne particles. In addition, the useful lifetime of these objects may be limited to a small number of restorations, the frequency of which is determined, at least in part, by the rate of aerosol deposition.
In this context, the problem of relating the rate of deterioration of the visual quality of an object to the nature of its environment has two major components. One addresses the transport of particles from air to the surface; the other considers the interaction of the deposited particulate matter with the visual information transmitted from the object to the observer. The deposition of particles onto surfaces obviously leads to degradation of the visual qualities of the object.
In a different vein, work in clean rooms, including large-scale integrated circuit manufacturing, as well as other manufacturing processes, is increasingly impeded by deposition of air or fluid-borne particulate matter. While workers in the field are keenly aware of the problem, a more radical solution is required than what has generally been proposed and discussed in the art.
In this respect, G. B. Larrabee (1985), in an article entitled "A challenge to chemical engineers--Microelectronics," Chemical Engineering (June 19), 51-59, gave an overview of the microelectronics manufacturing process, commenting that "There is much to learn about particles and their control in semiconductor device manufacture."
Douglas W. Cooper (1986), in an article entitled "Particulate Contamination and Microelectronics Manufacturing: An Introduction," Aerosol Sci.Technol. 5:287-299, described the general problem of controlling particle contamination in the microelectronics industry.
Abstracts by Locke et al. and by Peterson (1986) in Aerosols--Formation and Reactivity, proceedings Second International Aerosol Conference, 22-26 September 1986, Berlin, illustrate that the problem of particle deposition in the semiconductor industry is recognized to be an important one and demonstrates that aerosol scientists are working to understand the deposition process.
B. Y. H. Liu and K. Ahn (1987), in an article entitled "Particle deposition on semiconductor wafers," Aerosol Sci. Technol., 6, 215-224 provided a theoretical analysis of particle deposition onto semiconductor wafers assuming forced laminar flow.
N. Schafer and D. A. Kotz (1987), in an article entitled "Successful clean room design," ASHRAE J., (September), 25-28, indicated the level of effort undertaken in the semiconductor industry to minimize airborne particle concentrations, recommending better clean room design.
Fluid circulation patterns in rooms are often driven by natural convection. Although many researchers have investigated the deposition of particles on surfaces, none have addressed the theoretical aspects of particle deposition onto a vertical isothermal surface in a natural convection flow. The combined effects of turbulence, Brownian motion, and gravitational settling on particle deposition in enclosures have been investigated theoretically by Corner, J. and Pendlebury, E. D. (1951), "The coagulation and deposition of a stirred aeorsol," Proc. Phys. Soc. (London), B64, 645, and Crump, J. G. and Seinfeld, J. H. (1981), "Turbulent deposition and gravitational settling of an aerosol in a vessel of arbitrary shape," J. Aerosol Sci., 12, 405, and experimentally by Crump, J. G., Flagan, R. C. and Seinfeld, J. H. (1983), "Particle wall loss rates in vessels," Aerosol Sci. Technol., 2, 303, and Okuyama, K., Kousaka, Y., Yamamoto, S. and Hosokawa, T. (1986), "Particle loss of aerosols with particle diameters between 6 and 2000 nm in stirred tank," J. Colloid Interface Sci., 110, 214.
That work has recently been expanded to account for the effects of electrostatic forces, of particular interest for experiments conducted in Teflon-film smog chambers by McMurry, P. H. and Grosjean, D. (1985), "Gas and aerosol wall losses in Tef smog chambers," Envir. Sc. Technol., 19, 1176, and McMurry, P. H. and Rader, D. J. (1985), "Aerosol wall losses in electrically charged chambers," Aerosol Sci. Technol., 4, 249. The particle loss rate due to deposition on chamber surfaces under natural convection flow conditions has been investigated experimentally by Harrison, A. W. (1979), "Quiescent boundary layer thickness in aerosol enclosures under convective stirring conditions," J. Colloid Interface Sci., 69, 563.
The surface accumulations of ionic substances have been related to indoor concentrations to determine the rates of particle deposition in a room, as apparent from Sinclair, J. D., Psota-Kelty, L. A. and Weschler, C. J. (1985), "Indoor/outdoor concentrations and indoor surface accumulations of ionic substances," Atmos. Envir., 19, 315.
The deposition loss rate has also been studied experimentally by Scott, A. G. (1983), "Radon daughter deposition velocities estimated from field measurements," Hlth Phys., 45, 481, and Toohey, R. E., Essling, M. A., Rundo, J. and Hengde, W. (1984), "Measurements of the deposition rates of radon daughters on indoor surfaces," Rad. Prot. Dosim., 7, 143, and theoretically by Schiller, G. E. (1984), "A theoretical convective-transport model of indoor radon decay products," Ph.D. thesis, Department of Mechanical Engineering, University of California, Berkeley, for unattached radon decay products, which are believed to exist as very small particles with effective diameters in the range 0.001-0.01 .mu.m.
The influence of thermophoresis on particle migration near surfaces also has been investigated by Watson, H. H. (1936), "The dust-free space surrounding hot bodies," Trans. Faraday Soc., 32, 1073, Zernik, W. (1957), "The dust-free space surrounding hot bodies," Br. J. Appl. Phys., 8, 117, Goren, S. L. (1977), "Thermophoresis of aerosol particles in the laminar boundary layer of a flat plate," J. Colloid Interface Sci., 61, 77, Talbot, L., Cheng, R. K., Schefer, R. W. and Willis, D. R. (1980), "Thermophoresis of particles in a heated boundary layer," J. Fluid Mech., 101, 737, and Batchelor, G. K. and Shen, C. (1985), "Thermophoretic deposition of particles in gas flowing over cold surfaces," J. Colloid Interface Sci., 107, 21.
The definitive solution to the problem of heat and momentum transport to a vertical isothermal plate in a natural convection flow was reported by Ostrach, S. (1953), "An Analysis of laminar free-convection flow and heat transfer about a flat plate parallel to the direction of the generating body force," NACA Report 1111, U.S. Government Printing Office, Washington, D.C. Because of the analogy between heat and mass transfer, the extension of this solution to the deposition of highly reactive dilute gases is straightforward. However, for particles, because transport is influenced by factors in addition to advection and Brownian motion, the analogy does not hold.